Address CI failures due to flexcache

[khaeru]

This mirrors changes made in khaeru/genno#179 to address hgrecco/flexcache#6 / IAMconsortium/units#54. The IAM_UNITS_CACHE environment variable (available since iam-units 2025.10.13) is set to a different directory for each pytest-xdist worker. This prevents collisions.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A; CI changes only.
• Update release notes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.4%. Comparing base (12cf19e) to head (5b4dd8f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #982   +/-   ##
=====================================
  Coverage   96.4%   96.4%           
=====================================
  Files         60      60           
  Lines       5135    5138    +3     
=====================================
+ Hits        4954    4958    +4     
+ Misses       181     180    -1     

Files with missing lines	Coverage Δ
message_ix/testing/__init__.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[khaeru]

The failure Exception: Could not properly read symbol […] from GDX file. Try setting read argument mode='string' appears:

• macos-latest-py3.12, test_feature_price_commodity.py::test_commodity_price[ixmp4], with "symbol all_modes"
• windows-latest-py3.12, test_cap_comm.py::test_new_params_working[ixmp4-kwargs2], with "symbol fix_cost"

I'll treat these as unrelated to the current PR, just noting for visibility.

[khaeru]

@glatterf42 I'm also noticing that the job macos-latest tutorials seems to have many tests timing out. I am not sure if the cause is related; perhaps we can merge this PR and then start a separate one to address that issue.

[glatterf42]

LGTM, thanks for this fix :)